The present invention relates generally to improvements in blast furnace operations, and more specifically to methods and materials for improving the quality of blast furnace coke.
Investigations of blast furnaces that produce less hot metal than expected has shown that coke at the level of the furnace tuyeres was smaller in size, less permeable to air flow, more readilly abraided, higher in alkali content, and more reactive with CO.sub.2 than the coke fed into the blast furnace. In coke CO.sub.2 reactivity tests that were conducted, the "tuyere" coke resulted in more coke fines than production coke. These poor quality characteristics result from the catalytic effect of the higher amounts of alkali contained in "tuyere" coke on increasing the coke reactivity, promoting coke degradation, and subsequently increasing the resistance to air passage.
Alkalis, mainly, sodium and potassium, are carried into the blast furnace stack as constituents in the iron burden, the flux and the coke. For example, the burden may contribute up to 28% or more of the inputed K.sub.2 O and from 60 to 76% or more of the inputed Na.sub.2 O, and the coke may contribute from 64 to 73% of the K.sub.2 O and from 19 to 32% of the Na.sub.2 O. Depending upon the slag basicity and high temperature, part of these alkalis are removed with the slag, primarily as alkali silicates. Because of the reducing and high temperature conditions existing in the bosh region of the furnace, the remainder of the alkalis are vaporized as oxides or as metal vapors and are carried with the ascending bosh gases to the upper stack where a portion of the vaporized alkalis are condensed on the relatively cool descending materials, including the coke, charged into the furnace.
It has been shown that the alkalis that deposit on the descending coke charge appreciably accelerate the solution loss reaction on the coke-carbon reaction (C + CO.sub.2 = 2CO). It has also been demonstrated that, in direct relation to increased coke reaction, the reacted coke exhibits an increased tendency to degrade into fines that tend to plug the void spaces in the bosh zone so as to impede the flow of ascending hot reducing gases and descending liquids, i.e. slag and hot metal. This reduces the furnace productivity and causes a tendency for the furnace to hang.